This invention relates to projection video systems and specifically to projection video systems utilizing a light valve of the type known as a deformable mirror device.
Most commercially available projection video systems utilize separate projection paths for each of the three primary colors. The systems thus require red, green and blue light valves and optical paths which must be accurately converged on the screen which adds to complexity and expense. Recently, projection video systems utilizing only a single light valve have been developed. One such system is a color field sequential system, in which the normal video field, 1/60th of a second, is broken into three parts, or color subfields of 1/180th of a second.
During the three color sub-fields, the light valve is illuminated with red, green and blue light sequentially. While the light valve is illuminated with any given color, the video data corresponding to that color is displayed on the light valve. The eye then fuses the three color sub-fields into a single, full color field. The eye also fuses successive video fields into full motion, full color video. This system requires a device for sequentially illuminating the surface of the light valve with the three colors. The simplest of such devices is a color wheel which changes the color of a white projection lamp as it rotates.
Recently, improved light valves particularly suitable for use in projection television systems have become available. One such device is a so-called deformable mirror device (sometimes called a digital mirror device or DMD) which is illustrated in U.S. Pat. No. 5,079,544 (the disclosure of which is hereby incorporated by reference as if fully set forth herein) and patents referenced therein, in which the light valve consists of a array of tiny pivotable mirror-like pixels for deflecting a beam of light either to the display screen (on) or away from the display optics (off). This device is particularly suitable for use in a field sequential system because its pixels are capable of being switched very rapidly. By additional rapid switching of the pixels a grey scale is generated.
There is always a desire for greater brightness in projection video systems so that the brightness approaches or exceeds that of direct view (CRT) systems. In addition to a bright image, the image must also have good contrast so that the projected image does not appear "washed out". The present invention is directed towards providing an illumination system that has both increased brightness and increased contrast. A usual method of increasing brightness in a projection video system is to increase the aperture of the system. However, increasing the aperture generally increases the complexity and expense of the optical system and may reduce system contrast because the increased aperture permits more spurious light to enter the system. Where high contrast is of paramount importance many projection designs have decreased the aperture to increase the contrast, but at the expense of reduced brightness.
The present invention is adapted to the unique geometry of the DMD system in order to increase the brightness of the system without adversely affecting the contrast of the system. As in any projection system, the illumination system comprises a light path extending from a projection lamp to the projection lens. In this invention, an asymmetric aperture is located at one or more places along the light path. The asymmetric aperture is oriented such that the longitudinal axis of the asymmetric aperture is aligned with the pivot axis of each of the pixels (the longitudinal axis of the asymmetric aperture is thus orthogonal to the switching direction of the 10 pixels). If greater contrast enhancement is desired, the aperture can be further arranged so that it is narrower in the direction in which light is diffracted or scattered from the structure of the DMD. The asymmetric aperture can be disposed in the light path between the lamp and the DMD or after the DMD (i.e., in the projection lens) or at both places. The invention is useable in color or monochrome systems as well as systems using multiple light valves and multiple projection lamps.